The field of fiber optics is generally concerned with the transmission of light along a transparent fiber structure or core which has a higher refractive index than its surroundings. Typically, an optical fiber consists of a core of transparent material having a refractive index n1 surrounded by a layer of transparent cladding material having a refractive index n2 which is lower than n1. The core and the cladding form a guiding region. Usually, the core and guiding region have a circular or elliptical shape. Fibers with elliptical cores have a minor and major axis such that light travels along one of these axes. Such a fiber is called a polarization-holding fiber. The optical fiber can also include an outer protective layer.
Currently, it is possible to manufacture long, continuous strands of optical fiber which may propagate signals without substantial attenuation over long distances. It is also possible to manufacture the fiber structure as an optical waveguide wherein only preselected modes of light propagate in the fiber. By limiting wave propagation through the fiber to a single mode, the bandwidth of the optical fiber may be exceedingly high to provide a high information-transfer capacity.
In sensing and control systems, a fiber-optic transducer is used that exploits either multi-mode or single-mode light propagation in an optical fiber. While multi-mode sensors use amplitude variations in the optical signals to sense and transmit the desired information, single-mode sensors use phase variations rather than amplitude variations. The single-mode sensors usually involve mechanisms for altering such properties of the fiber as path length or index of refraction to effect the desired phase variations in the optical signal. Thus, in contrast to multi-mode sensors, in single-mode sensors the uniformity and mechanism of light propagation and hence the characteristics of the fiber are especially critical.
Single-mode sensors are sensitive to the state of polarization of the light in the fiber. Thus, for single-mode transducers, it is desirable to use elliptical-core or other kinds of polarization-holding fiber. See, e.g. McMahon et al, "Fiber-Optic Transducers," IEEE Spectrum, December, 1981, pages 24-27. Most of these polarization-holding fibers are capable of preserving the polarization of signals along two different, usually orthogonal, axes, such as the major and minor axes of an elliptical core.
As is known in the literature, e.g., Dyott et al., "Preservation of Polarization in Optical-Fiber Waveguides with Elliptical Cores," Electronics Letters, Jun. 21, 1979, Vol. 15, No. 13, pp. 380-82, fibers with elliptical cores and a large index difference between the core and cladding preserve the polarization of fundamental modes aligned with the major and minor axes of the ellipse, i.e., modes having their electrical fields parallel to the major and minor axes of the ellipse. If the core-cladding index difference and the difference between the lengths of the major and minor axes of the ellipse are sufficiently large to avoid coupling of the two fundamental modes, the polarization of both modes is preserved.
Commonly, it is necessary to splice together optical fibers. Optical fibers are spliced together by aligning two similar fiber ends, bringing the two fiber ends into contact and bonding the two ends together. Bonding is accomplished by fusing or melting the two fibers together, mechanically holding the fibers together or adding a bonding agent in the interface between the two abutting surfaces of the fiber ends. Optical fiber splices suffer from two types of attenuation losses. Intrinsic losses arise from differences between the two fibers being joined, such as differences in the refraction index and the size and shape of the fiber core. Extrinsic losses occur from the misalignment of the fiber ends and contamination.
In a typical fusion splicer for optical fibers, the two fibers that are to be spliced to each other are laid in a holding "chuck," which consists, at a minimum, of a metal rod with a precision machined V-groove and a clamping mechanism to hold the fibers in place. The clamping mechanism is often a flat spring with a small weight on top. In that case, the fiber must protrude above the top of the V-groove so that the spring contacts the fiber. The clamping mechanism can also include the application of a vacuum to the bottom of the V-groove.
Active alignment techniques ensure proper alignment of the two fibers by launching light into the fiber and detecting the light at the other end. Polarization-holding fibers such as elliptical core fibers require that the major and minor axes of the core properly align. If the two fibers do not properly align, then an alignor moves one fiber in two orthogonal directions orthogonal to the fiber axis until a maximum amount of light is detected. Once aligned, the gap between the fiber ends is then decreased by moving one fiber in an axial direction. The whole process is repeated until an absolute maximum amount of light appears at the detector. Another alignment method is described in U.S. Pat. No. 5,323,225. After alignment, the holding chucks keep the fibers aligned by maintaining the position of the optical fibers relative to each other.
These holding chucks are essential for positioning optical fibers in order to align and splice the fibers together. The "chuck" components are usually machined from metal and, while softer than the silica-based fiber, can damage the fiber surface. This damage results from particles (from the machine tools) becoming embedded in the machined chuck components. These particles can scratch the fiber surface and initiate a fracture of the fiber.